FIG. 1 is a cross-sectional view of a conventional gate electrode of a semiconductor device, which comprises a semiconductor substrate 1, a semiconductor active layer 2 disposed on the substrate 1, and a gate electrode 4 having a gate length Lg in a recess 3 formed in the surface of the active layer 2.
The gate electrode 4 having a cross-sectional structure as shown in FIG. 1 may be formed in the following manner.
As shown in FIG. 2, the semiconductor active layer 2 is formed on the semiconductor substrate 1, and a photoresist layer 30 is disposed on the semiconductor active layer 2. Then, by means of a photolithographic technique, such as electron beam lithography, the photoresist layer 30 is patterned to have an opening 5 formed therein. The opening 5 extends to the active layer 2. The width t.sub.1 of the opening 5 determines the gate length Lg of the completed gate electrode 4 shown in FIG. 1.
Next, as shown in FIG. 3, the recess 3 having a desired depth and width is formed in the active layer by, for example, wet etching, using the photoresist layer 30 as a mask.
After that, as shown in FIG. 4, a gate electrode metal is deposited by, for example, vacuum vapor deposition to form a gate electrode metal layer 60 on the photoresist layer 30. When the gate electrode metal layer 60 having a thickness h is formed on the photoresist layer 30, a layer 61 of the gate electrode metal having a thickness substantially equal to h is also deposited in the recess 3. As the gate electrode metal is vapor deposited, a small amount 62 of the metal is also deposited on an upper portion of the opening 5 in the photoresist layer 30, so the opening 5 becomes smaller and smaller. Accordingly, the amount of the gate electrode metal passing through the opening 5 gradually decreases, which results in tapering of the gate electrode metal layer 61 formed in the recess 3 as shown.
Then, the photoresist layer 30 and the gate electrode metal layer 60 on the photoresist layer 30 are lifted off, which leaves the gate electrode 4 in the recess 3 as shown in FIG. 1.
It is known that one factor which determines device characteristics, such as a noise figure (NF), of, for example, FET's is the gate length Lg, and that as the gate length Lg is smaller, the NF is improved. However, as the gate length Lg becomes smaller, the cross-sectional area of the gate electrode 4 may also become smaller, which, in turn, increases gate resistance and, hence, degrades the NF. Accordingly, in order to reduce the length Lg of the conventional gate electrode and still increase its cross-sectional area for preventing the gate resistance from increasing, the thickness or height h must be increased. However, for the gate length Lg of less than about 0.3 .mu.m, the thickness of the photoresist layer 30 cannot be increased in view of patterning and lift-off processing. This imposes a limitation on the height h of the gate electrode 4, and, it is impossible to use a larger height h in order to increase the cross-sectional area of the gate electrode 4. For this reason, a semiconductor device with a gate electrode having a structure as shown in FIG. 1 cannot have an improved NF.
A gate electrode having a reduced gate length Lg and having a large cross-sectional area has been proposed in, for example, Japanese Published Patent Application No. SHO 63-273363. The gate electrode shown in the Japanese application has a T-shaped structure. With the gate electrode structure shown in this Japanese application, however, dielectric films, such as a silicon oxide film and a silicon nitride film, are present in the space between the gate electrode and the active layer, which result in undesired parasitic capacitance.
Another technique for reducing the gate length Lg of a gate electrode is shown in Japanese Published Patent Application No. SHO 63-95676. The gate electrode structure disclosed in this Japanese patent application is aimed only to reducing the gate length, and it is not contemplated to reduce both the gate length and the gate resistance.
Japanese Published Patent Application No. SHO 63-155771 shows a semiconductor device with a T-shaped gate electrode. However, the invention of the Japanese application has been contemplated for displacing the T-shaped gate electrode toward a source electrode from the center of the distance between the source and drain electrodes of the device. This Japanese application does not specifically mention the reduction of the gate electrode length and the reduction of the gate resistance.